Looping through a generator that returns multiple values

Question

My generator ( batch_generator ) returns 5 values, but I can't seem to figure out how to loop through the values.

Things I've tried:

1) Unpack directly in the for loop definition ( ValueError: too many values to unpack (expected 5) )

for a, b, c, d, e in next(batch_generator):
    # do something with a-e

2) Unpack within the for loop ( ValueError: too many values to unpack (expected 5) on the line where I unpack item )

for item in next(batch_generator):
    a, b, c, d, e = item
    # do stuff

3) Zip it and unpack in the for loop definition ( ValueError: not enough values to unpack (expected 5, got 1) )

for a, b, c, d, e in zip(next(batch_generator)):
    # do stuff

4) Zip it and unpack within the for loop ( ValueError: not enough values to unpack (expected 5, got 1) on the line where I unpack item , I think it's just wrapped in another tuple now)

for item in zip(next(batch_generator)):
     a, b, c, d, e = item

Any explanation of what's really going on with the tuples/generator would be appreciated!

Answer 1

My yield statement looks like yield a, b, c, d, e

Based on that comment, the generator seems to emit a sequence of 5-tuples.

You can then simply use:

for a, b, c, d, e in batch_generator:
    #                ^ no next(..)
    pass

So you should not use next(..) . Next simply returns the next yield . Now since that is a tuple, the for loop will iterate over the tuple , instead of the generator.

The for loop will iterate over the tuples the generator batch_generator emits, until the generator is exhausted (or there is a break / return statement in the for loop that is activated.

Mind that a for loop works like:

for <pattern> in <expression>:
    # ...

The <expression> should be an iterable (generator, tuple, list,...) and the <pattern> is used to assign to. If you iterate over the tuple, you thus iterate over the elements of that tuple, not the tuple in full.